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New submitter Doug Otto sends word that researchers working on the ALPHA experiment at CERN are trying to figure out whether antimatter interacts with gravity in the same way that normal matter does. The ALPHA experiment wasn't designed to test for this, but they realized part of it — an antihydrogen trap — is suitable to collect some data. Their preliminary results: uncertain, but they can't rule it out. From the article:
"Antihydrogen provides a particularly useful means of testing gravitational effects on antimatter, as it's electrically neutral. Gravity is by far the weakest force in nature, so it's very easy for its effects to be swamped by other interactions. Even with neutral particles or atoms, the antimatter must be moving slowly enough to perform measurements. And slow rates of motion increase the likelihood of encountering matter particles, leading to mutual annihilation and an end to the experiment. However, it's a challenge to maintain any antihydrogen long enough to perform meaningful experiments on it, regardless of its speed. ... The authors of the current study realized that [antiatoms trapped in ALPHA] eventually escaped or were released from this magnetic trap. At that point, they were momentarily in free-fall, experiencing no force other than gravity. The detectors on the outside of ALPHA could then determine if the antihydrogen was rising or falling under gravity's influence, and whether the magnitude of the force was equivalent to the effect on matter."

Maybe our universe is a 'matter bubble' in a 'sea of anti-matter'. WE are the anti-matter.

To us, our normal matter is so common but that's only because we're sitting right smack in the middle of it. That would explain the repelling forces and show why dark matter could exist outside the bounds of the observable universe.

Not necessarily. If antimatter falls up, it would imply a repulsion effect between normal mass and antimatter mass. If we are a matter bubble we wouldn't be floating in antimatter, we'd likely be repelling it with a gap between it and us.

And the longer the universe exists, the greater the gap, and the greater the gap, the "faster" our portion of the universe seems to expand. Yay pseudo-science!

OK, then consider the minimum supersymmetric extension of the spherical cow model [wikipedia.org], in which all spherical cows have supersymmetric 'scow' partners. In practice, this should allow to work out similar results to the unpaired spherical cow model in the low energy cow scattering regime, while preserving the cow-pairing symmetry that you prefer.

No, there is no negative mass, and no FTL travel as a result. What you have if antimatter falls up is a change in reaction to a potential.

The defining character of mass is not gravity, as that is merely a potential which exists when you have two or more massive objects or a massive object and a photon. The defining character of mass is momentum. As such, in order for antimatter to fall up, it must inherently have mass, but that mass reacts to the potential of gravity by being repelled.

Sure, for many calculations, using a negative mass number will make the vector equations work out correctly for Newtonian dynamics and Galilean translations involving matter / antimatter gravitational interactions.

Relativistic mass adjustments will need to use the E^2 - (pc)^2 = (mc^2)^2 equation or simply be redefined as the magnitude of mass (minor notational changes really).

It would seem that antimatter could only fall up, if there was some way to distinguish gravitational and inertial mass. From my experience of how electrons and positrons were accelerated at SLAC, their inertial mass was identical. The only difference between them was their charge.

It would seem that antimatter could only fall up, if there was some way to distinguish gravitational and inertial mass. From my experience of how electrons and positrons were accelerated at SLAC, their inertial mass was identical. The only difference between them was their charge.

This is why it is important to conduct the experiment to see if the gravitational and inertial mass of antimatter are the same. Sure, we know that they're the same thing for ordinary matter and that antimatter and matter have the same inertial mass, but the effect hasn't been properly studied for antimatter (because that's a furiously difficult experiment). It could be that gravitational and inertial mass are the same for AM; that would be the most likely expected case, and we wouldn't learn that much about new physics if that's true. But we haven't checked, and so we must do so to make sure. After all, if they were different that would be a really important fact about the universe that we are currently unaware of. (It would be far more important than finding the Higgs boson.)

Let the experiment be done. Let us find out if the universe is even stranger than we thought it was. It's this sort of thing that a fundamental physics lab should study.

How is this any different from how *the entirety of physics terminology* is generated? "This is a handy way to talk about the mathematical systems that match our observations" is basically where every physics term comes from. There isn't some "more real" definition for the terms *we made up* to describe the universe than what stuck after some physicist decided to use it.

If they have created hydrogen atoms already, why wouldn't they just check to see if those atoms fall to the bottom of the container, or float to the top? I would guess these atoms are stored in a vacuum, so buoyancy isn't a concern.

No, hydrogen atoms in a vacuum are going to be more effected by nearby weak magnetic fields then by gravity. And even more so by what direction they where traveling. So you'd first need to slow them down which is hard to do when all you can use is magnetic force. Of course you could try to accurately measure there trajectory at two or more points and then figure out how gravity effected the atom. But it's hard to get real good measurements of atomic trajectories with strong magnetic fields. If we could mak

Actually measuring them accurately is a challenge, although no one in the physics community really expects the answer to be "they fall up" at this point. It would be a huge upset if they did.

There's a (possibly apocryphal) story about a physics professor. Whenever he dropped his chalk, writing equations on the board, he would look upwards. When one of the students finally asked him why he did this, he replied, "If one day it fell upwards, I wouldn't want to miss it."

Actually, I'm pretty sure a lot of them would have the opposite reactions. When the Higgs Boson was finally found, a lot of physicists were actually disappointed because it meant there wasn't really much in the way of new physics to be discovered.

Well, in all fairness, nobody was really sure about the Higgs. A lot of people were hoping, but nobody was willing to bet the farm on the mass (or even the existence of the Higgs).

Now, gravity on, the other hand, is a completely different aniamal.

IANAP (I am not a physicist), but the way that I understand it is that gravity is simply following a straight line in curved space-time. So, a straight line is a straight line for both matter and anti-matter. If anti-matter flies up, then that totally blows the

See, I think they should fall up. Antiparticles are predicted by the negative energy solutions of the Dirac equation.

But they still have positive energy. (Think of them as "holes" in a sea of negative-energy electrons; kick an electron out of that sea and you get a positive-energy negatively-charged electron and a positive-energy positively-charged "hole", i.e . a positron.)

Because these are individual atoms. Very hard to detect unless they are clumped together as a mass... as in the millions. The only way to know their position is to force them to be where you want them via a magnetic field... etc... which ruins your chances of measuring any gravitational effect which is unfathomably tiny at atomic scales. You could make a whole pile of them (very difficult indeed) so it would act more like classical matter... the problem there is that by the time you had that much, when it hit the bottom of your container you'd find out just exactly what e=mc2 is all about and likely need to start looking for a new research facility.

But you get a nice annihilation event whenever they touch matter. That makes it much easier.

So you constrain it in a a known location with an electric field (or two, a DC and an AC field, like an electrodynamic balance) in a vacuum, then let go and see where the two photons from the annihilation come from. With a large enough vessel and a sensitive PET-like setup, you should be able to tell whether it hit the top or the bottom of the vessel.

Pft. That's just an engineering problem. You have plenty of charged matter with known properties to test it on (though the detector will of course be different). Also simple stuff like turning the device upside down will help suss out instrument bias. At 0.6% equatorial vs polar gravity, you could likely tell a difference in travel time even with a bias in the trap.

Its difficult because for a single atom gravity is very weak. Small magnetic or electric fields (or field gradients) can interact with the magnetic field, or electric dipole moment of the atom. Also the atoms are moving inside of the trap. The speed of their motion depends on temperature: (at room temperature it is > 1 kilometer/second). I assume they cool the anti-hydrogen, but the atoms may still be moving so quickly that gravitational effects are not very large.

I'm an ion trapper, and though I don't work on this experiment, I've heard their group leader speak on exactly this topic a year or so ago, so hopefully I can do it justice from memory.

There are a couple challenges. One is "letting go". The atoms are trapped by very strong magnetic fields, and those have to be turned off rapidly to "let go" of the atoms. They turn off the superconducting magnet coils by heating them above their critical temperatures to make them normal-conducting and dumping all that energy into heat ("quenching" the magnet). Then the atoms are free to move around, but they weren't just sitting perfectly still in the traps, they had some thermal motion, which could fling them in any direction, including up. They've had trouble getting the atoms as cold as they had planned. They hoped they would be around 3 K, but I think they were stuck at 10 or 20 K for some unknown reason. So they aren't really just "dropping" the atoms. More atoms will go down than up if they are affected by gravity as expected, but it isn't remotely universal. Additionally their current trap is horizontal because the beam comes in from that direction, so there are only a few vertical cm in which to build up that bias.

Perhaps the bigger issue is actually knowing which way the atoms went. Their current trap was designed to do laser spectroscopy of atoms sitting in the trap, not tracking atoms as they fly around the beamline. What they do is wait until an anti-atom hits a surface and annihilates with a normal atom, and detect the radiation that is released from the annihilation. The radiation flies off in every direction though, so it takes some doing to build a radiation detection array that can reconstruct where in the apparatus the annihilations actually take place. As I mentioned, the current trap was not optimized for this particular study, so the reconstruction ability is pretty weak.

They are working on building the next generation of the experiment that will include a vertical trap, better detection arrays, and colder atoms, so that should be able to get to a better detection.

When you put a "normal" gas in a container, room temperature imparts speeds on the atoms that would take several kilometers to reach the top of a parabolic trajectory assuming no other interaction with container walls or other gas molecules. In a distance of one meter the difference of speeds would only be about 40 ppm at best for such molecules, and the mean free path is much shorter.

If you put any gas though into a container such that the mean free path is longer than the distance it takes for signific

Obviously there must be some credence to this idea for such an experiment to take place, but since my understanding is that gravity is an inherent effect of mass warping space, wouldn't anti-matter possess mass in the same way that matter does, so why would gravity act differently?

Obviously there must be some credence to this idea for such an experiment to take place, but since my understanding is that gravity is an inherent effect of mass warping space, wouldn't anti-matter possess mass in the same way that matter does, so why would gravity act differently?

Just asking. Not trying to claim anything.

Inertial mass and gravitational mass are observed - for normal matter - to be exactly equivalent. There's no actual reason they should be though, since they're the product of very different interactions - it's perfectly logical to have something which "weighs" a 1000kg when experiencing electromagnetic acceleration, and only 10kg when experiencing gravitational acceleration.

For normal matter, this is the case. For antimatter it's presumed but not actually tested, and therein lies the rub. Even a slight deviation would be huge - and have big implications for the question of why the universe has so much matter in the first place.

Inertial mass and gravitational mass are observed - for normal matter - to be exactly equivalent. There's no actual reason they should be though, since they're the product of very different interactions - it's perfectly logical to have something which "weighs" a 1000kg when experiencing electromagnetic acceleration, and only 10kg when experiencing gravitational acceleration.

The discussion is about mass, not weight. Weighing something is a very indirect way to determine mass; but regardless, it's about mass, not weight. If it were about weighing schemes a term other than mass would have been used.

That is, inertial mass determines how much an object will be accelerated by a particular force.

Gravitational Mass comed from Newton's law of graviation:

F = G * m_g1 * mg2 / r ^2

That is, the magnitude of the gravitational forces between two objects.

The question is whether the two definitions of mass are interchangable (e.g. does m_i = m_g1?). That appears to be the case for normal matter, which we can tell because all objects accelrate at the same rate

Inertial mass and gravitational mass are observed - for normal matter - to be exactly equivalent. There's no actual reason they should be though, since they're the product of very different interactions

Well, if you believe General Relativity, they darn well better be equivalent. In fact, Einstein took the Equivalence Principle as one if his starting points when developing GR. If the Equivalence Principle fails (which it must if anti-matter falls up), then they will have disproven Einstein's theory, which would be very big news, indeed.

Inertial mass and gravitational mass are observed - for normal matter - to be exactly equivalent. There's no actual reason they should be though, since they're the product of very different interactions

Well, if you believe General Relativity, they darn well better be equivalent. In fact, Einstein took the Equivalence Principle as one if his starting points when developing GR. If the Equivalence Principle fails (which it must if anti-matter falls up), then they will have disproven Einstein's theory, which would be very big news, indeed.

Which would hence be the value of a test that it is in fact enforced. Again: we can only assume it's true because the laws we know work in other cases assume it's true. But there's no implicit reason to think that we aren't simply observing a whole lot of local cases where some higher principle is simplifying to General Relativity, or where at the fringes there's a small correcting constant which isn't significant in most normal situations.

This type of measurement is where new physics comes from - it's why there's people who have been measuring alpha to ever greater precision, even though we've no reason to think it'll deviate if current theory is a complete explanation.

Looking at just the object's reaction to EM fields, you couldn't. But you could also observe its effects on charged bodies of known mass and charge, which would provide you with enough data to deduce the reality.

Incidentally, it's not a question of "inetia agains X fields" - it's simply that there are two quantities we call mass - inertial mass, which resists acceleration by *any* force, and gravitational mass, which creates and reacts to a gravitational field ("gravitational charge"). There are theoretical reasons why inertial mass will be constant regardles of the nature of the force, but no accepted explanation for why gravitational and inertial mass maintain a fixed ratio in all observed phenomena.It is *presumed* that antimatter has positive gravitational mass because the existence of negative g-mass particles would have some really wierd consequences, the possibility of perpetually accelerating machines not the least of them. A positive g-mass with a different ratio to inertial mass would be unexplained by current theory, but wouldn't really break things - we still don't actually have a very good theory as to the source of gravitaional mass effects.

If they changed the definition of antimatter to "has an anti-higgs-boson-field" such that the matter actually has an inverse affect on the universe, that might cause it to have "anti-mass" which would warp the anti-space referenced by the anti-field. However, this is not how we have traditionally defined anti-matter; the original definition was actually due to the fact that the universe has significantly less mass than it should, and "anti-matter" was hypothesized as an explanation. So by definition, anti

However, this is not how we have traditionally defined anti-matter; the original definition was actually due to the fact that the universe has significantly less mass than it should, and "anti-matter" was hypothesized as an explanation.

Actually, the original modern definition of anti-matter was "Dirac's relativistic equation for the wave function of the electron had negative energy states as well as positive energy states, which was a bit weird, so it was proposed that all the negative energy states were filled, and if you knocked an electron out of one of the low-energy states, a "hole" would be left behind, and that hole behaved like an electron, except that it has a positive charge". It was later seen in the real world (particles moved in a magnetic field as if they had the mass of an electron and a +1 electrical charge). See, for example, the Wikipedia article about the positron [wikipedia.org].

All theory says that anti-matter should behave like matter becasue both kinds of matter have positive energy (hence the release of energy when they annihilate). Negative energy is harder to produce than anti-matter, and it is possible that there are fundamental limits to its production. Negative energy would produce anti-gravity (at least I think, given my rudimentary knowledge of GR). As "Electricity Likes Me" said in his/her reply, it is *possible* that contrary to what is expected, anti-matter floats

In general relativity, gravity is a warping of space and EVERYTHING falls at exactly the same speed. This has been tested to very high accuracy in a variety of experiments. (see eotvos experiment)

There are other theories of gravity where this doesn't necessarily need to be true and antimatter and matter might fall at different rates. The eotvos type experiments have indirectly tested this since there is some amount of virtual antimatter in normal objects (from quantum fluctuations), but a direct measurement

If antimatter interacts with gravity in such a way that it "falls" up or pushes against the force like magnetic fields pushing against each other, does this mean that antimatter would make anti-gravity platforms possible?

I'm a science plebe who watches/reads too much sci-fi, this was the first thing that came to my mind.

However, a thorny engineering problem would be stopping the tons of antimatter holding up the platform from interacting with the normal matter around it. If it did, *boom*. That's the way you can get 100% of the "E" out of the "m" in E=mc^2.

Absolutely. Of course a tiny floating 100kg platform would require 100kg of antimatter, and I wouldn't want to be anywhere nearby if your EM-containment field failed. If it were a solid chunk of anitmatter it probably wouldn't be *too* bad - a small surface explosion when it contacted the platform followed by a hail of antimatter meteors falling upwards while blasting out gamma rays from air-molecule collisions. A tank of anti-H though, that would mix very quickly and release all the energy at once - and

Much (most?) of the energy from an ordinary nuclear bomb comes off as gamma rays. Because the atmosphere happens to be relatively opaque to gamma, it absorbs them and superheats. That's what generates the fireball.

So, expect the same thing to happen with antimatter.

And actually pure gamma emission is what happens when electrons and positrons collide. Proton-antiproton collisions tend to produce gamma plus some secondary particles (pions (pi-mesons), if I remember right, but I may not).

It would mean that we have a preliminary report on an unfinished experiment. Or more specifically, an experiment not intended to explore this subject has not ruled out the possibility.

What this actually means to us is that experiments intended to find this result have not been proven useless already, and they could be conducted using the existing ALPHA setup. ALPHA appears to be the most successful anti-particle creation mechanism, making it the obvious plac

we already know antimatter doesn't have "negative mass" in that sense, it responds with expected inertia to acceleration by electromagnetic forces. we already know the yield of annhilation too (relativistic mass is positive). question is just of response to gravitational field of normal matter, which way the force vector points.

Right, we know it has positive inertial mass. We haven't yet properly observed their gravitational mass. We assume the two are equivalent; they may not be.

Actually, physicists have antimatter all wrong. A positron actually does have a negative charge but also has negative inertial mass, so it will react to an electromagnetic field the opposite way an electron does. We just observe that as reversed charge.

Based on what we currently know, we would expect that the only significant force acting on a piece of falling antimatter is gravity; by the equivalence principle, this should make antimatter fall with the same acceleration as ordinary matter. However, some theories predict new, as yet unseen forces: these forces would make antimatter fall differently than matter. But in these theories, antimatter always falls slightly faster than matter; antimatter never falls up. This is because the only force that would treat matter and antimatter differently would be a vector force (mediated by the hypothetical gravivector boson). Vector forces (like electromagnetism) repel likes and attract opposites, so a gravivector force would pull antimatter down toward the matter-dominated Earth, while giving matter a slight upward push.

The question of whether anti-matter experiences anti-gravity goes back as far as I can personally remember (1970's) and probably some decades before that.

For most of the past 300 years in physics, experiment has led theory. We measure something, it leads to a theory, and then experiments are done to check the theory. Examples abound of theories that explain previous observations, and also predict something new - probably the most famous is relativity predicting the precession of Mars, but there are lots of others. (Newton predicting elliptical orbits based on the inverse square law of gravity comes to mind.)

Since about 1970 the situation is reversed - theory has led experiment. We have a satchel of theories which attempt to explain questions in physics which have no discriminatory evidence. Theories such as "Super Symmetry", "Loop Quantum Gravity", and "String Theory". I'm reading a book right now [amazon.com] which claims 10^500 different string theories (yes, that's 10 with 500 zeroes after it), and lamenting the fact that few of these actually make testable predictions.

Relativity predicts that anti-matter should have positive gravity, but this has never been tested.

Until recently, the only antimatter we had access to has been charged particles: anti-protons and anti-electrons. Measuring the gravitational force on a charged particle is nigh impossible because the EM force is so large (relative to the gravitational force) that any EM effects swamp the readings. You can't just see if the particle falls in the container, because it's essentially impossible to shield a container well enough. It's like trying to measure the mass of a cork floating in a tornado.

Anti-hydrogen would work, but until recently we had none to test. Antiparticles tend to have high velocities when produced - they have to escape their anti-nemesis which is also produced - so they have to be slowed down enough to "pair" to make the neutral antimatter particle.

The vacuum used for the experiments has a big effect also. Depending on the level of vacuum used, any particle has a "mean free path [wikipedia.org]" before it will impinge on another particle. You have to get your anti-particles to slow down, form antimatter, and conduct the experiment before another particle comes in and annihilates it. This requires insanely good vacuum which is both hard to achieve and highly expensive.

The ALPHA [web.cern.ch] experiment at CERN now produces antimatter, so the referenced paper asks the question: what is the ratio "F" between the inertial mass and the gravitational mass of antihydrogen? For normal matter it's 1 and for "antigravity matter" it would be -1.

The paper reports that they have measurements within specific confidence levels that F < 110 almost certainly, and F < 75 at the 95% confidence level.

If the experiments outlined in the paper are continued (and perhaps refined), over time they can statistically narrow the results and ultimately settle the question by experiment.

I think that this would be a good thing, it would confirm (or contradict) by experiment something that is predicted by theory.

If antimatter is gravitationally repulsed by matter, then it could help explain dark matter. Instead of requiring a huge expansion of the Standard Model, it may simply be that the vacuum is gravitationally polarized.

Faster than light travel is only impossible when you have a net positive mass. If your mass is net zero, (meaning in your magnetic grip you hold matter and antimatter in the same functional unit but not touching each other (two magnetic bottles), then you could travel faster than the speed of light.

Anti-matter still has a positive mass. Otherwise when a positron meets an electron it wouldn't release any energy. Personally, I highly doubt it "falls up", as that would be inconsistent with general relativity because anti-particles would not follow a curved space. What would be really cool is if it was found that anti-matter curved space in the opposite direction as matter, making gravity repulsive. I highly doubt that's the case, but it would certainly be a cool discovery.

No, I understand that this particular story was about the reverse effects of antimatter.
I was taking the excuse of this news to point out the idea of negative mass being the ideal way to circumvent E=MC2. If your net mass is zero, then the massive energy that would be converted into mass does not need to happen. I agree with your assessment of what kind of mass antimatter has--positive mass--but if there were negative mass, that's the way to Warp 5.

... not that I don't think masturbation (and math, possibly at the same time) is nifty. Science is distinguished from mathematics in that it not only considers mathematical models, but compares (and judges) said models by correspondence to "real world" observations. Game theory, while "physically motivated," is mathematics. Game theory can be applied by scientists to explain real-world measurements, but on its own is not science.

By which standard mathematics manifestly is not science. While there is no "one true formulation" of what the "scientific method" exactly is, pretty much every formulation roughly follows the schema described here on Wikipedia [wikipedia.org], including the critical element of:

Testing: This is an investigation of whether the real world behaves as predicted by the hypothesis.

A mathematician --- let's consider a game theorist for example --- will set up a problem ("given this rigorously-defined hypothetical scenario, what strategy would result in maximization of mathematical object 'X'?"), and apply mathematical logic to

Exactly. Like evolutionary biology. Now that we've successfully created life from basic proteins and selectively applied evolutionary pressure until we get modern man hundreds of time, we have a solid scientific theory. And cosmology. Now that we have successfully created the universe hundreds of times and consistently observed its behavior, we have a high-confidence model for what it will do in the future.

And a good thing, too. Can you imagine where we would be if those types of things were inherently not

You'd of course need enough antimatter to balance the weight of the car. Let's call it 1500 kg of antimatter per car. Multiply by 2 for two cars and by 2 again for the mass of normal matter gives 6000 kg total being annihilated. That has an energy equivalent of 5*10^20 joules, which per wikipedia [wikipedia.org] was the total world energy consumption in 2010. This is also equivalent to about 10^5 megatons of TNT or 2000 Tsar Bombas.

1. Hydrogen rises in gravity because it is less dense than air(mostly Nitrogen), So if there was no air/vacuum then hydrogen would fall towards the earth.?

I can't say anything to the other two questions, but this question is easily answered by something my high school physics teacher said to me. It has stayed with me since then as it is as eye-opening as it is obvious (in hindsight):

"The first mistake is to assume that helium rises. The truth is that it falls down towards the earth just like any other object. The reason for what you see is much simpler: It does not rise; it's just that everything else simply falls harder." (Freely translated from memory and German)

Helium only rises over the air, because regular air has the stronger draw to be below it. This explains why, in the absence of gravity; there is no lift. In the absence of a pull, the air has no impulse to displace the helium.

More generally, the same is true for liquid mixtures like oil/water. In gravity, the oil will rise above the water. In (close-to-)zero gravity, the oil and water will separate but stay where they are. That is because the water can't displace the oil without gravity pulling it more strongly down.

The same is true for solids. In meteorites with too little gravity, no submersion of the "heavier" elements like iron happen. This is why Earth has an iron core, but iron-rich asteroids have it distributed all over their volume.

1. Hydrogen rises in gravity because it is less dense than air(mostly Nitrogen), So if there was no air/vacuum then hydrogen would fall towards the earth.?

I can't say anything to the other two questions, but this question is easily answered by something my high school physics teacher said to me. It has stayed with me since then as it is as eye-opening as it is obvious (in hindsight):

"The first mistake is to assume that helium rises. The truth is that it falls down towards the earth just like any other object. The reason for what you see is much simpler: It does not rise; it's just that everything else simply falls harder." (Freely translated from memory and German)

Helium only rises over the air, because regular air has the stronger draw to be below it. This explains why, in the absence of gravity; there is no lift. In the absence of a pull, the air has no impulse to displace the helium.

More generally, the same is true for liquid mixtures like oil/water. In gravity, the oil will rise above the water. In (close-to-)zero gravity, the oil and water will separate but stay where they are. That is because the water can't displace the oil without gravity pulling it more strongly down.

The same is true for solids. In meteorites with too little gravity, no submersion of the "heavier" elements like iron happen. This is why Earth has an iron core, but iron-rich asteroids have it distributed all over their volume.

That is true for a container (e.g. a balloon*) filled with helium. AFAIK it would also be true for individual helium atoms if the temperature was 0 Kelvin. They would basically fall like little rocks.

In reality the Earth is actually warm enough and light enough that unconfined helium atoms frequently reach escape velocity and fly away into space. They do this often enough that a cloud of helium will never settle on top of the atmosphere like oil on top of water. It will just diffuse into space. There is no

1) Not exactly.. In the absense of other forces all known objects/particles will accelerate towards each other due to gravity. Hydrogen rises for the same reason a boat floats in water - buoyancy. Denser things (like water or air) fall "harder" and push less-dense things (like boats or hydrogen) out of the way. When you release a piece of wood underwater it doesn't fall up, it gets lifted up by the denser surrounding water which experiences a larger gravitational force per unit volume. It's basically an